With the requirements of improving integration, miniaturization, high frequency and fast response for the electronic device as well as the demand for IR (infrared rays) image arrays, it is imperative for the sensor device to be fabricated as a micro-system. Therefore, techniques for the nano/micro-electro-mechanical system (NEMS/MEMS) are increasingly improved on the mentioned purpose.
Regarding the conventional micro-thermistor (thermal resistor), such a thermal resistance device is advantageous for its low cost and its ability for being available at a room temperature and for being fabricated as a non-contact device. In comparison with the capacitance thermaistor, the thermal resistance device is also superior in the response speed. Therefore, people not only in the academia but also in the industrial have done a great effect on the fabrication of micro-thermistor arrays.
The most principal factor for dominating a thermistor is the temperature coefficient of resistance (TCR). The definition of TCR is given as TCR=dR/(dT×R), wherein R represents the resistance of the thermistor, and dR and dT represent the differential of the resistance and the time period, respectively.
Depending on the magnitude of TCR, there are two types of typical thermal resistance materials which are commonly used in the thermistor, including a PTCR (positive TCR) material and an NTCR (negative TCR) material. The PTCR material always has a TCR value of only about +2%, while the TCR value of the NTCR material is always larger. However, the NTCR material may damage the thermistor due to the effect of thermal runaway, and hence a complicated feedback circuit for compensation is always an essential portion therein which is disadvantageous for the thermistor to be miniaturized and integrated. Besides, it is also necessary for the thermistor to possess a larger TCR value while it is miniaturized. Therefore, it has become a serious issue for the skilled person that how to obtain a PTCR material with a high TCR value.
Please refer to FIG. 1, which is a side view schematically illustrating a conventional micro-thermistor device according to the prior art, in which the micro-thermistor device is fabricated by means of a typical surface micromachining technique. The micro-thermistor device 1 includes a thermistor 10 which is fabricated from a suspended flake and is so-called a sensing pixel. Each sensing pixel may detect infrared rays from a heat source and be further heated thereby, and the resistance of the sensing pixel is varied therewith, accordingly. While the resistance variation resulting from the heat is detected, the temperature or the heating state of the heat source is derivable from a pre-constructed database. The thermistor 10 is fabricated as a suspended structure to prevent the gathered heat from being affected by the heat conduction and from dissipating in a short time. Furthermore, the specific detectivity and the responsibility are also critical factors for the thermistor 10 and can be optimized by means of the improved fabrication
Ferroelectric materials with a perovskite structure, such as LiTaO3, BaTiO3 and (Ba, Sr)TiO3, are commonly used in the bolometer application, while the thin film microsensor mainly adopts a pyroelectric element or a thermal capacitor. Such a thin film micro thermistor is disadvantageous for its slow response, large interference and poor signal detectivity.
By contrast, the mentioned drawbacks might be overcome by the micro-thermistor array fabricated from a high PTCR material. For example, the polycrystalline strontium-barium titanate (Ba, Sr)TiO3, i.e. the BST material, exhibits an extremely high PTCR at the curie temperature TC. More specifically, the PTCR is almost higher than 20% which is extremely higher than that of the conventional metallic PTCR materials, so that the polycrystalline BST material performs as an excellent material for the themistor. Furthermore, BST is a semi-insulating material with a band gap ranged in 2.5˜3.5 eV, which is convertible into a semiconducting material exhibiting a high electric conductivity by the dopant implementation, and thus the BST material is suitable for the thermistor application.
Nowadays, relevant techniques and efforts are developed on bulks or powders of the perovskite-type material and the PTCR mechanism thereof is known as the Heywang model. At the TC temperature, the perovskite-type material would transform from the ferroelectric phase to the paraelectric phase, which results in a rapid variation of the polarization thereof. While the temperature is increased, the depletion region on the grain boundary in the perovskite-type material may enlarge rapidly, so as to equalize the trapping charges thereon, and the resistivity of the perovskite-type material is increased due to the increment of the boundary potential resulted therefrom.
As for the thin film of the perovskite-type material, however, the TCR value thereof may be dramatically reduced by the inevitable interface stress even to a range of NTCR. Therefore, it is an important issue for the thin film BST-thermistor that how the interface stress could be reduced and how the TCR value could be enhanced.
On the other hand, a common scheme for enhancing the PTCR of the BST material is by doping some hetero-elements such as chromium Cr and manganese Mn therein and followed by a step of being sintered at a temperature higher than 1000° C. Such a scheme is disadvantageous because the thermal stress resulting from the high temperature would seriously damage the film property and makes it impossible to drive-in the dopants into the film material. Moreover, with respect to the physical coating techniques, it is also difficult to perform Cr or Mn pre-doping in the BST target since the melting point as well as the molecule bonding of the metal Cr or Mn is hugely different from that of the ceramic BST.
In order to overcome the above drawbacks in the prior art, the present invention provides a novel microsensor with a ferroelectric material and a novel fabrication method. The provided microsensor is processed by means of the micro-electro-mechanical system (MEMS) in which the dopant Cr or Mn is driven in the BST film via a microwave post-treatment or an excimer laser annealing treatment so as to fabricate a Cr-doped or an Mn-doped BST film for enhancing the TCR thereof.